Exhaust streams from combustion sources, particularly from diesel engines, may be required to have particulate filters located within the exhaust system to clean the exhaust stream prior to discharge to the atmosphere. In order to provide an effective long-life for such filters, it is necessary to occasionally remove the collected particulates especially since too great an amount can clog the filter and create undesirable back pressure, which in the case of diesel engines, is most undesirable.
Conventionally, such as shown in U.S. Pat. No. 4,211,075, this is accomplished by a regenerator system which ignites the stored particles, burning them off, and thus cleaning the filter. The majority of such systems act to raise the exhaust temperature to a level at which the particulates ignite. This is done primarily either by throttling the entire air intake or by blocking whole sections of the engine to render them inoperative or by igniting a pilot burner to actively raise the exhaust gas temperature. Other examples of known systems are shown in U.S. Pat. No. 4,270,936 and German Offenlegungsschrift No. 2756570 which both use electric heaters to raise the temperature of the filter.
Techniques relating to throttling the intake air of combustion sources, primarily engines, while capable of increasing exhaust gas temperatures, also creates undesirable and detrimental problems particularly because the throttling itself leads to oxygen deficient operation of the engine during the regeneration cycle. This, in turn, can lead to uncontrollable temperatures during the particular burning stage. I have observed that at the various rpm and load points that there are particular points of throttling where beyond that point the engine would "stumble" and emit large quantities of CO, HC and particulates. This "stumbling" phenomenon appears to be always accompanied by oxygen levels in the exhaust gases which are below the minimum required for filter regeneration.
According to the present invention, a method and apparatus are provided for regenerating catalytically coated particulate filters, such as the type of filter shown in copending U.S. patent applications Ser. No. 161,873 filed June 23, 1980 (now abandoned) and refiled on Dec. 29, 1982 as Ser. No. 448,277, and Ser. No. 55,403 filed July 6, 1979 (now abandoned) and refiled on Sept. 30, 1982 as Ser. No. 429,423; and such as shown in published British applications Nos. 2,024,646 and 2,054,402A. It is desirable to provide regeneration of such filters without effecting a substantial rise in the exhaust gas temperature. Rather, ignition and burn off of the stored particulates is effected, according to the present invention, by controlling the content of the exhaust from the engine and most particularly by controlling the exhaust emitted by the plurality of cylinders in that engine, either grouped into pairs or individually. By so doing, the combined exhaust from the cylinders can be effectively regulated so that there exists in the exhaust stream, at the point the various exhaust streams are mixed together, sufficient "fuel" for the filter's catalyst to generate an exotherm. In turn, that exotherm will ignite the particulates or at least create hot spots adjacent or within the collected particulates with the subsequent ignition in either case serving to clean and thus "regenerate" the filter.
The catalyst coating on catalyzed filters develops an exotherm when oxidizable fuels, such as carbon monoxide and gaseous hydrocarbons, in the presence of sufficient oxygen are presented to the catalyst above the temperature at which the catalyst is active, typically above 200.degree. C. The exotherm being generated thereby will depend upon the quantity of the "fuel" being supplied and also the catalyst activity.
The present invention may be practiced with any catalyzed particulate filter associated with an exhaust stream from a combustion source. However, the invention is particularly applicable to exhaust streams from diesel engines. The present invention is primarily practiced by presenting to a catalyzed filter gaseous fuel emissions from the combustion source at a temperature that is above the filter's gaseous "light-off" temperature. The "light-off" temperature of a catalyzed filter differs from filter to filter and is different for each species of material (gas, particulate, etc.) to be oxidized. The appropriate and respective "light-off" temperature may be determined experimentally. For instance, a portion of a catalyst may be heated gradually raising the temperature together with particulate matter collected from the exhaust gas stream of a combustion source in the sample pan of a differential scanning colorimeter in an atmosphere of 1% oxygen and argon. Samples of the atmosphere above the sample pan are taken via a heated capillary tube to a mass spectrometer. Four mass numbers are traced, (1) carbon monoxide, (2) doubly charged argon, (3) oxygen and water or nitrogen, and (4) carbon dioxide. The temperature at which the differential plot of the differential scanning colorimeter peaks is taken to be the temperature at which the combustion of the particles took place, and this temperature is the "light-off" temperature for the ignition of the particulate.
The present method comprises the steps of: during operation of the combustion source, presenting to the catalyzed filter above its gaseous emission light-off temperature gaseous exhaust containing sufficient "fuel", sufficient to generate an exotherm so that the catalyzed filter heats up and ignites stored particulates in the filter and in contact with the catalyst over a period of time sufficient to burn up a substantial quantity of the particulates previously collected and stored in the filter.
According to one primary aspect of the present invention, the method and apparatus provide an efficient way to increase oxygen levels in the exhaust stream at or just prior to the "stumbling" point. This is accomplished by "pulsing", or only intermittently operating the throttle system according to a predetermined operating cycle. In general, throttling, according to the present invention, would be applied leading to the "stumbling" point and the high gaseous emissions with appropriate controls to remove the throttling effect which in turn leads again to increased oxygen content in the exhaust. The resulting high gaseous and oxygen exhaust streams flowing from respectfully controlled cylinders are mixed in the exhaust system prior to contact with the catalyst, with the mixed gases comprising the "fuel" for regeneration purposes.
The pulsating of the intake air step of the method herein may be practiced periodically, either manually or automatically, in response to a variety of sensed conditions or combinations thereof. Such conditions preferably include but would not be limited to engine revolutions or backpressures caused by the filter itself. When regeneration is practiced, it is preferably practiced for a preset time interval, of at least several seconds, so that a substantial amount of the stored particulates are burned up and the filter regenerated.
The pulsating of the intake air step may be practiced in a number of different ways. One way is to employ a multi-point control system which, for an engine having a plurality of cylinders each having its own intake manifold line, modifies those intake manifold lines by placing a small butterfly valve in each. This enables the engine to be throttled on a cylinder by cylinder basis, permits very quick response times and enables high pulsation speeds or cycles. Specifically, with reference to a four cylinder engine and numbering the cylinders, 1, 2, 3 and 4, from front to rear, the two central cylinders, 2 and 3, can be controlled as a pair independently of cylinders 1 and 4. By controlling the butterfly valves of cylinders 2 and 3 to the point where the valves are rapidly cycled through closed and open conditions, with those cylinders becoming emitters of a low oxygen content exhaust rich in CO and HC. Cylinders 1 and 4 on the otherhand, can be operated normally so that they produce a normal, oxygen rich exhaust. Such an operating mode separates the rich gas from the high oxygen gas by half of one engine revolution with the flow of exhaust within the filter providing sufficient mixing for the exhaust from cylinders 1-4 to generate a good exotherm quickly on the catalyst once the "fuel" comes in contact with the catalyst itself.
Another way is to connect each of the four butterfly valves together for cylinders 1-4 so they can be simultaneously activated, but to control the extent to which valves for cylinders 1 and 4 can close, by means of appropriately placed stop that will limit value closure. In this manner the valves controlling intake air flow for cylinders 1 and 4 will only be partially throttled during each pulsation within the cycle. In addition to butterfly valves, rotating vanes could also be used with vanes rotating through 360.degree. in accordance with a predetermined but regulatable rpm.
According to the present invention, an apparatus for practicing the different aspects of the method according to the invention, as described above, is also provided. By practicing the present invention, it is possible to control the particulates emitted from a diesel engine in a vehicle to about 0.1 grams per mile, and to provide automatic regeneration of the filter without significant driver perception of the regenerator operation.
It is the primary objective of the present invention to provide a simple and effective method and apparatus for regenerating a catalyzed particulate filter in a combustion source exhaust stream. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.